The present invention is in a process and an agent for the cross-linking of organopolysiloxanes.
Depending upon the degree of polymerisation, organopolysiloxanes, either in the form of oils or as plastic solid masses in the form of silicone rubber, can be made up of siloxane chains with alkyl or aryl substituents R, such as methyl, phenyl or vinyl, on the silicon atom according to the following structural formula ##STR2##
The hydrogen atoms in the alkyl substituents can also be replaced wholly or partly by fluorine or chlorine atoms in order to achieve special chemical properties. As a rule, silicone rubber contains 1 to 10 vinyl radicals per 1000 silicon atoms.
These silicone rubbers are sometimes provided with pigments or filling materials in order to influence their color and strength. However, silicone products with a high transparency and without impurities and colorations are usually desired, particularly for medical and foodstuff use in tubes, seals, profiles, foils and the like.
For the production of these products, continuous processes are usually employed and vulcanisation is preferably carried out in an infra-red hot canal at 150.degree. to 600.degree. C. without pressure and in the presence of air.
For this use, industrial chlorinated bis-benzoyl peroxides, such as bis-(2,4-dichlorobenzoyl)peroxide, have hitherto proved useful as cross-linking agents. However, this compound gives rise to hazardous decomposition products of the polychlorinated biphenyl (PCB) type. Other peroxides, for example dicumyl peroxide, cannot be used since they give rise to bubbles in the vulcanisate, discolorations of the desired transparent article or also a drastic reduction of the productivity due to too slow decomposition.
After vulcanisation, the silicone rubber products are normally subjected to treatment with hot air at 150.degree. to 250.degree. C. for several hours in order to free them from volatile products of the siloxane production process and of the peroxide decomposition. Furthermore, the optimum mechanical properties (tensile strength, E-module, hardness, compression set) are only achieved after this treatment. However, in the case of this tempering, the silicone article must not become discolored.
Because of the increased official requirements in the case of the use of chlorinated aromatics, there is an urgent need for a replacement for bis-(2,4-dichlorobenzoyl)peroxide in the case of the use of which the chlorinated decomposition products of the latter, such as 2,4-dichlorobenzoic acid and especially 2,4-dichlorobenzene and bis-(2,4-dichlorophenyl) cannot arise (Rubber World, August, 1985, p. 12 et seq.).
Therefore, it is an object of the present invention to find an appropriate chlorine-free organic peroxide, especially for the pressureless vulcanisation of silicone rubber, which does not suffer from the disadvantages of the chlorinated peroxides.
Benzoyl peroxide itself is unsuitable since it produces bubbly vulcanisates when pressure is not used.
Furthermore, the sought after peroxide should also fulfil the following conditions:
be storage-stable up to +30.degree. C. for several months (preferably 3 to 6), PA0 be miscible in silicone oil in a concentration of from 10 to 80%, PA0 give decomposition products which can be removed by tempering for several hours with hot air at 150.degree. to 250.degree. C. without discoloration of the vulcanisate, not losing its white color in silicone paste form during the storage since the vulcanisate then also becomes discoloured, PA0 display a self-decomposition temperature which is so high that official requirements do not make necessary a storage and transport with cooling.